The present invention relates to electrodeless discharge lamp operating apparatuses.
Electrodelss discharge lamps (or electrodeless low-pressure discharge lamps) have excellent characteristics including a resource saving effect of long life and an energy saving effect of high efficiency, so that electrodeless discharge lamps have been widely noted in the illumination field in recent years. Hereinafter, a conventional electrodeless low-pressure discharge lamp operating apparatus will be described with reference to FIG. 2.
FIG. 2 shows the structure of a conventional electrodeless low-pressure discharge lamp operating apparatus, and an electrodeless low-pressure discharge lamp operating apparatus having such a structure is disclosed in, for example Japanese Laid-Open Patent Publication No. 58-57254.
The electrodeless low-pressure discharge lamp operating apparatus shown in FIG. 2 includes a discharge vessel 21 enclosing a luminescent metal and rare gas therein, a phosphor 22 applied onto an inner face of the discharge vessel 21, and a coil 23 inserted in a recessed portion 21a of the discharge vessel 21. The phosphor 22 is used to convert ultraviolet rays occurring in the discharge vessel 21 to visible light. The coil 23 is constituted by a rod-shaped core 23a made of a magnetic material such as ferrite and a winding 23b. The rod-shaped core 23a of the coil 23 has a rod-shaped member 26 made of a thermal conductive material in its central axis (a hatched portion in FIG. 2). The rod-shaped member 26 serves to dissipate and suppress the heat of the coil 23 that is generated during lamp operation.
The discharge vessel 21 is supported by a metal case 25, and the rod-shaped member 26 provided inside the recessed portion 21a of the discharge vessel 21 and the metal case 25 are coupled to each other. In such a structure, it is intended to minimize the heat generation of the coil by dissipating the heat generated in the coil 23 from the metal case 25 through the rod-shaped member 26. A power source 24 for supplying high frequency alternating current to the winding 23b is provided in the metal case 25. In other words, the lamp is configured so that an alternating magnetic field occurs from the coil 23 by the high frequency alternating current from the power source 24. A lamp base 27 is attached to a part (a lower part) of the metal case 25.
Next, the operation of the electrodeless low-pressure discharge lamp operating apparatus shown in FIG. 2 will be described.
First, an alternating magnetic field is generated in the discharge vessel 21 from the coil 23 with the high frequency alternating current supplied from the power source 24 to the winding 23b. Then, an alternating electric field occurs in the discharge vessel 21 to cancel this alternating magnetic field. The luminescent metal and rare gas in the discharge vessel 21 collide with each other repeatedly so as to be excited by this alternating electric field so that a plasma is formed in the discharge vessel 21. Ultraviolet rays are radiated from the plasma and the ultraviolet rays are converted to visible light with the phosphor 22. Thus, the visible light is emitted outwardly from the discharge vessel 21. In this manner, the electrodeless low-pressure discharge lamp operating apparatus shown in FIG. 2 emits light.
In the above-described operation, the coil 23 is forced to operate at a quite high temperature caused by the heat generated by a loss due to the alternating current supplied to the winding 23b and the heat generated by heat conduction from the plasma. Furthermore, the recessed portion 21a in the discharge vessel 21 where the coil 23 is provided is constituted by a closed space, so that heat is unlikely to be dissipated, and therefore it is necessary to take some measure to dissipate the heat for the electrodeless low-pressure discharge lamp operating apparatus. Japanese Laid-Open Patent Publication No. 58-57254 describes as a measure for dissipation that the rod-shaped member 26 made of a thermal conductive material is inserted in the central axis of the rod-shaped core 23a, and the heat generated in the coil 23 is dissipated from the metal case 25 through the rod-shaped member 26 by coupling the rod-shaped member 26 and the metal case 25.
In the conventional structure as above, it is necessary to use a material having good thermal conductivity for the rod-shaped member 26, and it is generally assumed that a metal can be used. In the case of the rod-shaped member 26 made of a metal, an eddy current is generated in the rod-shaped member 26 by a magnetic field occurring in the coil 23, and thus a loss occur. Similarly, a loss due to the eddy current also occurs in the metal case 25. Therefore, in the above conventional structure, the generated eddy current reduces the lamp efficiency, and sufficient heat dissipation effects cannot be obtained. Furthermore, since this structure is very complicated where the rod-shaped member 26 of the central axis of the rod-shaped core 23a is inserted, and the rod-shaped member 26 and the metal case 25 are coupled, disadvantageously resulting in a large apparatus.
Furthermore, since a metal and rare gas are enclosed as the luminescent substance, the luminous flux is low during a period from turning on the power to evaporation of the metal, and the start of lighting takes time. In addition, the metal vapor pressure is significantly varied by the variation of the ambient temperature, so that the variation of the luminous flux disadvantageously is large. The variation of the metal vapor pressure leads to the variation of the electrical characteristics of the plasma, and therefore the power source 24 that can cope with a wide range of load variations can be of a complicated structure, disadvantageously resulting in a large apparatus. Moreover, mercury generally is used as the luminous metal to radiate ultraviolet rays. However, there is a great demand for reducing the amount of mercury used in view of environmental protection.
The present invention is carried out in view of the above problems, and it is a main object of the present invention to provide an electrodeless low-pressure discharge lamp operating apparatus that can suppress an increase of the temperature of the coil.
An electrodeless discharge lamp operating apparatus of the present invention includes a transparent discharge vessel in which a luminescent substance is enclosed; a coil for generating an alternating electromagnetic field that discharges the luminescent substance; a power source for supplying alternating current to the coil, wherein the coil comprises at least a magnetic material, and is disposed on an inner side than the outer side wall of the discharge vessel, and the luminescent substance comprises at least a rare gas, and does not comprise mercury.
In one embodiment, the coil is inserted in a recessed portion provided in the discharge vessel.
In one embodiment, a frequency of the alternating current supplied by the power source is in a range from 40 kHz or more and 500 kHz or less.
In one embodiment, the electrodeless discharge lamp operating apparatus further includes a phosphor applied onto an inner face of the discharge vessel, and ultraviolet rays occurring in the discharge vessel are converted to visible light with the phosphor.
In one embodiment, the luminescent substance is a rare gas, and the rare gas is at least one selected from the group consisting of xenon, argon, krypton, neon, and helium and a mixture of these rare gas.
It is preferable that the rare gas comprises at least xenon.
In one embodiment, a pressure in the discharge vessel before discharge start is in a range from 0.1 torr or more and 3.0 torr or less.